1. Field
Disclosed herein is a tire with a radial carcass reinforcement and more particularly a tire intended to be fitted to vehicles carrying heavy loads and driving at sustained speeds, such as, for example, lorries, tractors, trailers or buses.
2. Description of Related Art
In general, in tires of the heavy vehicle type, the carcass reinforcement is anchored on each side in the region of the bead and is surmounted radially by a crown reinforcement consisting of at least two superposed layers formed of threads or cords which are parallel within each layer and crossed from one layer to the next, making angles of between 10° and 45° with the circumferential direction. The said working layers, which form the working reinforcement, may even be covered with at least one layer known as a protective layer and formed of reinforcing elements which are advantageously made of metal and extensible, known as elastic elements. It may also comprise a layer of metal cords or threads of low extensibility making with the circumferential direction an angle of between 45° and 90°, this ply, referred to as the triangulation ply, being situated radially between the carcass reinforcement and the first crown ply known as the working crown ply, formed of parallel threads or cords making angles at most equal to 45° in terms of absolute value. The triangulation ply, together with at least the said working ply, forms a triangulated reinforcement which, under the various stresses it encounters, deforms very little, the triangulation ply having the essential role of reacting the transverse compressive forces to which all of the reinforcing elements in the crown region of the tire are subjected.
In the case of tires for “heavy” vehicles, just one protective layer is usually present and its protective elements are, in the majority of cases, oriented in the same direction and at the same angle in terms of absolute value as the reinforcing elements of the radially outermost and therefore radially adjacent working layer. In the case of tires for construction plant, intended to run over rather uneven terrain, the presence of two protective layers is advantageous, the reinforcing elements being crossed from one layer to the next and the reinforcing elements of the radially inner protective layer being crossed with the inextensible reinforcing elements of the radially outer working layer adjacent to the said radially inner protective layer.
Radially on the outside of the crown reinforcement is the tread which is usually made up of polymer materials intended to come into contact with the ground in the contact patch where the tire is in contact with the ground.
Cords are said to be inextensible when the said cords exhibit, under a tensile force equal to 10% of the rupture force, a relative elongation at most equal to 0.2%.
Cords are said to be elastic when the said cords exhibit, under a tensile force equal to the rupture load, a relative elongation at least equal to 3% with a maximum tangent modulus of below 150 GPa.
The circumferential direction of the tire, or longitudinal direction, is the direction corresponding to the periphery of the tire and defined by the direction in which the tire runs.
The axis of rotation of the tire is the axis about which it turns during normal use.
A radial or meridian plane is a plane containing the axis of rotation of the tire.
The circumferential median plane or equatorial plane is a plane perpendicular to the axis of rotation of the tire and which divides the tire into two halves.
The transversal or axial direction of the tire is parallel to the axis of rotation of the tire. An axial distance is measured in the axial direction. The expression “axially on the inside of or, respectively, axially on the outside of means “of which the axial distance, measured from the equatorial plane, is less than or, respectively, greater than”.
The radial direction is a direction intersecting the axis of rotation of the tire and perpendicular thereto. A radial distance is measured in the radial direction. The expression “radially on the inside of or, respectively, radially on the outside of” means “of which the radial distance, measured from the axis of rotation of the tire, is less than or, respectively, greater than”.
Certain present-day tires, known as “road” tires, are intended to run at high speed and over increasingly long journeys because of the improvements to the road network and the growth of the motorway network throughout the world. All of the conditions in which a tire has to run undoubtedly allow an increase in the distance that the tire can cover, because tire wear is lower; however, the endurance of the tire and particularly of the crown reinforcement is penalized.
This is because there are stresses in the crown reinforcement and more particularly shear stresses between the crown layers, combined with a not-insignificant increase in operating temperature at the ends of the axially shortest crown layer, which have the effect of causing cracks to appear and spread through the rubber at the said ends. This problem exists in the case of the edges of two layers of reinforcing elements, the said layers not necessarily having to be radially adjacent.
In order to limit excessively great increases in temperature in the crown of the tire, the materials of which the tread is made are advantageously chosen to be those that have hysteresis losses suited to the operating conditions of the tire.
Moreover, in order to improve the endurance of the crown reinforcement of the type of tire being studied, solutions relating to the structure and quality of the layers and/or profiled elements of rubber compounds which are positioned between and/or around the ends of plies and, more particularly, the ends of the axially shortest ply, have already been applied.
In order to improve the resistance to degradation of the rubber compounds situated near the edges of the crown reinforcement, Patent FR 1 389 428 recommends the use, in combination with a low-hysteresis tread, of a profiled element of rubber covering at least the sides and marginal edges of the crown reinforcement and made of a low-hysteresis rubber compound.
In order to avoid separation between crown reinforcement plies, Patent FR 2 222 232 teaches coating the ends of the reinforcement with a wad of rubber, the Shore A hardness of which differs from that of the tread surmounting the said reinforcement and which is higher than the Shore A hardness of the profiled element of rubber compound positioned between the edges of crown reinforcement and carcass reinforcement plies.
French Application FR 2 728 510 proposes positioning, on the one hand between the carcass reinforcement and the crown reinforcement working ply radially closest to the axis of rotation, an axially continuous ply formed of inextensible metal cords making with the circumferential direction an angle of at least 60° and the axial width of which is at least equal to the axial width of the shortest working crown ply and, on the other hand, between the two working crown plies, an additional ply formed of metal elements oriented substantially parallel to the circumferential direction.
French Application WO 99/24269 also proposes, on each side of the equatorial plane and in the immediate axial continuation of the additional ply of reinforcing elements substantially parallel to the circumferential direction, that the two working crown plies formed of reinforcing elements that are crossed from one ply to the next be coupled over a certain axial distance and then uncoupled using profiled elements of rubber compound at least over the remainder of the width that the two working plies have in common.
This improvement to the endurance of the tires means that it is possible at least to consider the option of re-treading, when the tread is worn. Specifically, when it is desirable to re-tread the tire after the tread has worn away, it is necessary, in order to optimise the use of the new tread, to be able to re-tread a tire that is not at too advanced a stage of ageing.
In order further to increase tire life, it is common practice to choose polymeric materials of which to make the tread that have improved wear resistance properties, and because such materials usually penalize the hysteresis properties, it is also known practice to create the tread of a tire as a radial superposition of two different materials in order to obtain a compromise between wear properties and hysteresis properties that is satisfactory for the envisaged applications.
Such tires are, for example, described in document U.S. Pat. No. 6,247,512. That document describes the superposition of two layers of materials to form the tread, the external material coming into contact with the ground notably performing better in terms of wear, whereas the internal material has hysteresis properties that make it possible to limit the increase in tire temperature in the crown region.
Tires produced in this way are entirely satisfactory for the above-mentioned road-going applications. In order to ensure the possibility of re-treading, it is, however, recommended that excessive degradation of the internal material be avoided, this material wearing relatively rapidly as compared with the external material, so as not to risk damaging the reinforcement of the tire.
As a result, the use of the tread of such tires cannot be fully optimized without the risk of compromising the option of re-treading the tire.